In a fibre optical communications system, it is important to be able to control the output power of the transmitting laser diode for a number of reasons. Firstly, the average and peak power of the laser must not exceed certain limits in order to avoid damage. Secondly, the different power levels corresponding to binary (or other radix) data values must be set so that the modulation index (alternatively defined as extinction ratio) is within the overall system specifications to ensure reliable reception at the end of the link. One difficulty to be addressed in any control system is that the characteristics of the laser can change significantly with temperature and also over time with ageing, and diverging from an ideal linear response, so that a conventional factory set up of the “high” and “low” drive current levels is not adequate.
Numerous techniques exist in prior art that describe methods intended to estimate the instantaneous values of the minimum and maximum transmitted optical output and compensate for the changes in device characteristics. Most are limited in their effectiveness due to the restricted bandwidth of the monitor diode and its associated circuitry.
Monitoring the transmitted output power is even more challenging in an optical communications link that transmits the data in a series of discrete bursts, as the average value of the optical output may vary greatly, and the instantaneous levels are not stable enough for most methods described in prior art to reach adequate estimates of minimum and maximum levels. The temperature related effects are likely to be even more severe, as the transmitting laser diode may be in an off state for a long period before being activated for a data burst, and hence may have cooled to ambient temperature before heating up during a data burst.
Hence it is desirable to be able to sense the minimum and maximum optical outputs corresponding to logic “1” and logic “0” during data bursts on a near continuous basis. It is further desirable to be able to make such measurements using a transmit power monitoring function with only moderate bandwidth, and by means that do not disturb the transmitted data payload nor compromise the received signal to noise performance.